Case studies

Two-dimensional (2D) materials such as graphene boast unique and powerful properties, however these properties will deteriorate if the material becomes contaminated. Luckily when different 2D materials are pressed together, stray molecules from the lab are pushed out leaving large flat areas clear of impurities. These clean regions have yielded some of the most fascinating physics of our time. Now, the assumption that these areas are completely clean is under scrutiny.

A team of researchers at The University of Manchester have shown that even the gas within which the 2D material stacks are assembled can affect the stack structure. They found that for the 2D transition metal dichalcogenides, some sheets had a large gap between them and their neighbour; a distance unexplained by theoretical calculations from collaborators at Radboud University, Netherlands. These observations seemed to point to the presence of impurities between the 2D sheets. To confirm this, 2D materials were stacked in a pure argon gas atmosphere using a glove-box. This time the distances matching those predicted by theory for a clean interface free from impurities. The consequences of this finding will directly impact on how we make and model graphene devices for future applications.

For more information, see the Faculty of Science and Engineering's InAbstract blog.

Scientists at The University of Manchester have fabricated highly miniaturised pressure sensors using graphene membranes which can detect minute changes in pressure with high sensitivity, over a wide range of operating pressures.

Writing in Nanoscale, Aravind Vijayaraghavan and recently graduated PhD student Christian Berger have shown that it is possible to make an atomically thin membrane of graphene float just nanometers above the surface of a silicon chip.

When pressure moves this membrane closer to the surface of the chip, the resulting change in capacitance is measured to read out the pressure change. By fabricating thousands of such floating membranes next to each other, a device can be made of exceptionally high sensitivity to pressure changes.

Graphene is the world's first two-dimensional material. The design takes advantage of its extraordinary thinness, combined with its high flexibility and the highest strength of any known material; a unique combination of superlative properties without which such a technology would not be possible.

So-called 'organic perovskites' have shown immense promise as cheap light harvesters for a new generation of solar cells, with device efficiencies rising in the last five years to more than 22% - now close to that of the conventional material, silicon. However, exploitation of these materials is currently prevented by their instability when exposed to water and air, which leads to rapid degradation. Understanding the mechanism of this reaction would allow researchers to identify methods by which the process may be retarded or even stopped. The difficulty is that the material goes off before the reaction of water at its surfaces can be studied.

Now a cross-disciplinary team of researchers from physics, chemistry and materials science at The University of Manchester has utilized a new technique, Near-Ambient-Pressure X-Ray Photoelectron Spectroscopy, to measure the reaction of pristine perovskite with water for the first time. The key was to manufacture the perovskite inside the spectrometer, without any contact with air. This allowed them to unambiguously determine the degradation mechanism. These results will help synthetic chemists to design new light harvesting materials, which will not easily decompose when exposed to water, so their full potential can be exploited.

For more information, see the Faculty of Science and Engineering's InAbstract blog.